The present invention relates to a PLL (phase locked loop) frequency synthesizer, and more particularly to a frequency synthesizer for use in a battery-powered apparatus, such as a portable radio transmitter/receiver.
There is a PLL frequency synthesizer involving a charge pump circuit, which will be described in detail later. The charge pump circuit is used for electrically charging and discharging a loop filter which supplies a control voltage to a voltage-controlled oscillator (VCO). As power voltage Vcc of the charge pump circuit is used the output voltage of a voltage regulator, which is +5 V for instance in a battery-powered portable radio transmitter/receiver. For this reason, the control voltage supplied to a VCO is limited within the range of 0 to 5 V, resulting in the disadvantage that the dynamic range of the oscillation frequency is correspondingly limited. Further because of the low control voltage, there arises another problem that the Q value of the variable capacitance diode of VCO is low, resulting in deteriorating the carrier-to-noise (C/N) ratio of the VCO output. Moreover, the loop filter takes a long time to be charged or discharged, and the frequency switching of the synthesizer also is slow.